A dental restoration piece-producing process typically provides for the use of two materials. Frequently, blended synthetic materials are applied to metal frames in order to make possible an aesthetically satisfactory dental restoration piece and to increase the biological compatibility of the dental restoration piece.
Blended synthetic materials are typically cured or hardened into a cured condition via irradiation thereof with light or heat or via a combination of light and heat. This results in an increase in temperature of the material. Problems then arise due to the difference between the heat expansion coefficients of the blended synthetic materials which have been applied onto metal frames and the heat expansion coefficient of the metal of the metal frames. While the blended synthetic material is typically still soft during the heating up phase—namely, in connection with a positive temperature gradient, the blended synthetic material hardens during the cooling off phase, whereupon stresses occur for the reason that the synthetic material contracts more than the metal during the cooling off phase.
Since the heat expansion coefficient of blended synthetic material is greater, hairline cracking occurs in the blended synthetic material either immediately after the polymerization, after the completion of production, or, possibly, also occasionally after the passage of a certain amount of time following production. Such cracking can lead to the accumulation of bacteria in usage.
Aside from the adverse hygienic considerations, another problem is that the collective securement of the blended synthetic material on the dental restoration piece is degraded by such stress cracking so that the danger exists that a portion of the blended synthetic material comes loose. Typically, the stress relief occurs as well at the thinnest locations of the dental restoration piece—for example, at interdental locations.
It has already been suggested, in order to prevent such problems, to accommodate or align the heat expansion coefficient of the synthetic material to the greatest extent possible to that of the deployed metal coatings. This approach, however, is subject to physical limitations as well as the functional requirement that polymeric synthetic materials must typically be deployed of the type which are curable via light and/or heat irradiation.
It has been suggested, in fact, to apply a compensation layer on the metal frame before the blended synthetic material is applied thereunto with the objective that the compensation layer will elastically or resiliently yield such that the occurrence of stress cracking in the contracting blended synthetic material will be avoided.
Such solutions are, however, technically problematical to realize in practice, are expensive, and, moreover, are apt to degrade the securement or attachment of the blended synthetic materials.